1. Field of the Invention
The invention relates to a power management system for enabling back-up batteries to supply peak and/or high current demand DC loads, such as DC motor starting or an uninterruptible power supply (UPS) used to power a critical load, such as, a data bus or other critical load after an event, such as a loss of primary AC or DC power, during relatively cold ambient temperatures. The power management system includes one or more heaters, for example, a low power heater and a high power heater. In a maintenance mode, when the data bus or other critical load is fed from a primary power source, the low power heater is used to maintain the battery packs at a predetermined temperature, such as the desired operating temperature, so that the batteries can provide their minimum required capability upon loss of the primary power source. In this mode, the battery charger powers the low power heater and also maintains the charge on the batteries. After an event, such as a loss of the primary power source, the battery temperature can become too low to enable the batteries to provide the required capacity to back up the critical load. During such a condition, the battery charger supplies power to one or both of the heaters as well as to the batteries. Since the capacity of the battery charger is normally insufficient to heat the batteries to an acceptable operating temperature in a relatively short period of time during such a condition, a portion of the residual power in the battery is used to boost the power to one or both of the heaters in order to speed up the time to get the battery to normal operating temperature.
2. Description of the Prior Art
Conventional systems which supply electrical power to critical loads, such as data buses or other critical loads, are known to be powered from an uninterruptible power system (UPS). Such UPS systems utilize a primary AC or DC power supply and a DC back-up power supply. Critical loads, such as data buses, are known to require DC power. As such, during normal operation, the AC power from the primary AC power supply is converted to DC by way of a converter and supplied to the critical load. Because of the criticality of the load, a back-up DC power supply is also selectively connectable to the critical load in the event of loss of the primary AC power supply.
It is important that the DC back-up power supply be maintained at virtually full capability at all times. Unfortunately, some back-up battery systems may be exposed to relatively cold ambient conditions after a loss of the primary power source. Depending on the ambient temperature, the back-up battery system may not be able to deliver full capacity DC current to the critical load. The reason for this is that the internal resistance/impedance of the battery is inversely proportional to the temperature of the battery. Thus, the internal resistance of the battery will be relatively high at relatively low temperatures preventing the battery from delivering its full capacity. In addition, as batteries age, the internal resistance of the batteries tends to increase causing the battery to provide less output current and capacity.
Because of the criticality of the load, different schemes have been provided to heat-up the back-up battery system when exposed to relatively cold ambient temperatures after a loss of the primary power source. Unfortunately, the primary power source can be unavailable for some period of time. During that time, the temperature of the battery can drop to a relatively low level. Once the primary power source becomes available, it is necessary that the battery capacity be sufficient to safely shut down the critical after a subsequent power loss, the required capacity. As such, the batteries are normally heated by the battery charger to a temperature that enables the batteries to deliver their rated capability. Unfortunately, the capacity of battery chargers in known systems do not have sufficient capacity to heat the batteries quickly. As such, the critical load must remain off-line until the batteries are heated to the desired operating temperature. Thus, there is a need to provide a system for heating up a battery to a temperature at which it can deliver its required current and capacity in a relatively short time in order to minimize the exposure of the critical load to a total loss of electrical power during a loss of the primary AC power system. There is also a need for compensating aging batteries so that such batteries provide a relatively constant performance over time,